1. Field of Invention
The present invention relates to an electronic timepiece and to a time difference correction method for an electronic timepiece that corrects the time difference based on satellite signals received from positioning information satellites such as GPS satellites.
2. Description of Related Art
The Global Positioning System (GPS) in which satellites (GPS satellites) orbiting Earth on known orbits transmit signals carrying superposed time information and orbit information, and terrestrial receivers (GPS receivers) receive these signals to determine the location of the receiver, is widely known. Electronic timepieces that acquire accurate time information (“GPS time”) from GPS satellites and adjust the current internally kept time to the correct time have also been developed as one type of GPS receiver.
GPS time is the Coordinated Universal Time (UTC) delayed by the UTC offset (currently +14 seconds). Therefore, in order for an electronic timepiece that uses the GPS system to display the current local time, the acquired GPS time must be corrected to the current local time by adding this time difference to the UTC, and information about the time difference to UTC must be acquired.
This electronic timepiece determines its current position in order to acquire the time difference information. However, if the signal reception level is too low, the orbit information cannot be correctly demodulated and the position can therefore not be calculated. As a result, the position is generally calculated only when the signal reception level exceeds a specific threshold value. However, if the location of the GPS satellite used for the positioning calculation is poor, the positioning calculation error becomes too great and the correct position cannot be determined. As a result, the position is generally only calculated if an index denoting degradation of the precision of the positioning calculation based on the current GPS satellite location is less than a specific threshold value. Therefore, if these threshold values are fixed and the reception level is below the threshold value or the index to the positioning calculation precision is higher than the threshold value, the position will not be calculated even if the position can be calculated.
A method of increasing the precision of the positioning calculation as much as possible while also increasing the likelihood that the position will be calculated by setting these threshold values high for the initial positioning calculation and then gradually relaxing these threshold values if the positioning calculation is unsuccessful has therefore been proposed.
However, the method taught in Japanese Unexamined Patent Appl. Pub. JP-A-2006-138682 takes time for the positioning calculation to converge in order to maintain the highest possible precision in the positioning calculation. Because power consumption increases as the time required by the positioning calculation increases, applying this method in electronic timepieces such as battery-powered wristwatches is difficult.